The present invention relates to a metal-plated aromatic polyimide film in which the plated metal is fixed to the surface of the aromatic polyimide film with high bonding strength.
An aromatic polyimide film shows high heat resistance as well as good electric characteristics, and are widely employed as material for producing various electronic devices. The aromatic polyimide film, however, has such a disadvantageous feature as poor adhesive property to an adhesive which is ordinarily employed for constituting electronic devices. Further, a metal film deposited on the aromatic polyimide film by sputtering shows poor adhesion to the polyimide film and is easily peeled off from the polyimide film. Therefore, studies for improving the poor adhesive property of an aromatic polyimide film have been made and reported.
U.S. Pat. No. 5,218,034 describes a polyimide film having improved adhesion and thermal durability containing from 0.02 to 1% by weight of tin based on the weight of the film.
U.S. Pat. No. 5,543,222 describes a vacuum metallized polyimide film comprising an aromatic polyimide layer containing a hydrocarbyl tin compound in oxidation state (II) or (IV) as an additive and metal plated layer bonded integrally with high bonding strength or high adhesion via a vacuum deposited metal layer without the use of an adhesive.
U.S. Pat. No. 5,272,194 describes a strengthened polyimide film having improved adhesion when bonded to a metal foil via a heat-resistant adhesive, containing from 0.02 to 1% by weight, based on the weight of the film, of an organo-metallic compound wherein the metal is tin, bismuth or antimony.
U.S. Pat. No. 5,227,244 describes a polyimide film having improved adhesion which is prepared by coating the surface of a partially cured or partially dried polyamide acid film with an organic solvent solution of a metal salt and heating the coated film to convert the polyamide acid to polyimide and dry the film. The metal salt is a salt of Sn, Zn, Cu, Fe, Co, Mn, or Pd. Thus treated surface can be combined with a copper metal foil via an acrylic resin adhesive.
Japanese Patent Provisional Publications 59-86634 and H2-134241 both describe that a polyimide film can be improved in its adhesion by plasma discharge processing.
Japanese Patent Provisional Publication H1-214840 describes a process for preparing a polyimide resin pattern which comprises the steps of forming a film of an aluminum alcoholate or a chelated aluminum compound on a film of polyimide or its precursor, coating a photoresist on the aluminum-compound layer, imagewise exposing the photoresist coat to light, and etching the light-exposed photoresist.
It is an object of the present invention to provide a metal-plated aromatic polyimide film in which a metal film is plated on the polyimide film with high strength bonding.
It is another object of the invention to provide a metal-plated aromatic polyimide film in which a metal film is plated on the polyimide film with high strength bonding which is produced easily in industry.
The present invention resides in metal-plated aromatic polyimide film comprising an aromatic polyimide resin film, a surface layer of which contains a palladium metal or a palladium compound dispersed therein, and a metal layer which is chemically plated on the surface layer of the polyimide resin film.
It is preferred that the metal-plated aromatic polyimide film of the invention further contains an aluminum metal or an aluminum compound in the surface layer of the aromatic polyimide resin film.
On the chemically plated metal-plated aromatic polyimide film of the invention, an electrolytically plated metal layer is easily formed.
The palladium metal- or palladium compound-containing surface layer of the polyimide film of the metal-plated aromatic polyimide film of the invention is preferably formed by placing a palladium compound on a self-supporting film of an aromatic polyimide precursor and heating the self-supporting film having the palladium compound thereon to a temperature such as 420xc2x0 C. or higher, at which the polyimide precursor is converted into its corresponding aromatic polyimide.
The palladium metal- or palladium compound-containing surface layer of the polyimide film is also favorably formed by coating or spraying a solution of a palladium compound on a self-supporting film of an aromatic polyimide precursor and heating the self-supporting film having the palladium compound thereon to a temperature such as 420xc2x0 C. or higher, at which the polyimide precursor is converted into its corresponding aromatic polyimide.
The palladium metal- or palladium compound-containing surface layer of the polyimide film is also favorably formed by extruding an aromatic polyimide precursor solution containing no palladium compound and an aromatic polyimide precursor solution containing a palladium compound simultaneously from a die and placing one extruded precursor solution on the other precursor solution to give a combined precursor solution film, and heating the precursor solution film to a temperature such as 420xc2x0 C. or higher, at which the polyimide precursor is converted into its corresponding aromatic polyimide.
In the metal-plated aromatic polyimide film of the invention, the plated metal film can be placed on one side or both sides of the polyimide film.
The polyimide film of the invention which has a surface layer containing a palladium metal or a palladium compound comprises a resin matrix of aromatic polyimide resin. The aromatic polyimide resin is preferably composed of an aromatic tetracarboxylic acid residue and an aromatic diamine residue. The aromatic tetracarboxylic acid residue is preferably derived from an aromatic tetracarboxylic dianhydride, and the aromatic diamine residue is preferably derived from an aromatic diamine. A small portion of the aromatic tetracarboxylic acid residue can be replaced with an aliphatic tetracarboxylic acid residue, and a small portion of the aromatic diamine residue can be replaced with an aliphatic diamine. The polyimide resin can contain an aminodicarboxylic acid residue such as 4-aminophthalic acid residue, 4-amino-5-methylphthalic acid residue, or 4-(3,3xe2x80x2dimethyl-4-anilino)phthalic acid residue, in addition to the aromatic tetracarboxylic acid residue and the aromatic diamine residue.
Examples of preferred aromatic tetracarboxylic acid dianhydrides include 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic dianhydride(s-BPDA), 2,3,3xe2x80x2,4xe2x80x2-biphenyltetracarboxylic dianhydride(a-BPDA), pyromellitic dianhydride, benzophenonetetracarboxylic dianhydride, and bis(3,4-dicarboxyphenyl)ether dianhydride (i.e., oxydiphthalic dianhydride).
Examples of the aromatic diamines include p-phenylene diamine and 4,4xe2x80x2-diaminodiphenyl ether. A portion of the aromatic diamine can be replaced with an aromatic diamine having plural benzene rings and a flexible structure such as 4,4xe2x80x2-diaminodiphenyl sulfide, 4,4xe2x80x2-diaminobenzophenone, 4,4xe2x80x2-diaminodiphenylmethane, 2,2-bis(4-aminophenyl)propane, 1,3-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 4,4xe2x80x2-bis(4-aminophenyl)diphenyl ether, 4,4xe2x80x2-bis(4-aminophenyl)diphenylsulfone, 4,4xe2x80x2-bis(4-aminophenyl)diphenylsulfide, 4,4xe2x80x2-bis(4-aminophenyl)diphenylmethane, 4,4xe2x80x2-bis(4-aminophenoxy)diphenyl ether, 4,4xe2x80x2-bis(4-aminophenoxy)diphenylsulfone, 4,4xe2x80x2-bis(4-aminophenoxy)diphenylsulfide, 4,4xe2x80x2-bis(4-aminophenoxy)diphenylmethane, 2,2-bis[4-(aminophenoxy)phenyl]propane, or 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, an aliphatic diamine such as 1,4-diaminobutane, 1,6-diaminohexane, 1,8-diaminooctane, 1,10-diaminodecane, or 1,12-diaminododecane, and other aromatic diamines such as xylylenediamine..
The aromatic polyimide resin preferably has a low linear expansion coefficient such as that in the range of 10xc3x9710xe2x88x926 to 25xc3x9710xe2x88x926 cm/cm/xc2x0 C., in the temperature range of 50 to 250xc2x0 C. The aromatic polyimide resin having such low linear expansion coefficient can be prepared from a combination of 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic dianhydride and p-phenylene diamine, a combination of a mixed aromatic tetracarboxylic acid components such as a mixture of pyromellitic dianhydride, 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic dianhydride and benzophenonetetracarboxylic dianhydride, and p-phenylene diamine (hereinafter referred to as PPD) alone or a mixed diamine components such as a mixture of PPD and 4,4xe2x80x2-diaminodiphenyl ether (hereinafter referred to as DADE). PPD and DADE are preferably employed in a molar ratio of 100/0 to 15/85 (PPD/DADE). The polyimide can be in any form of a homopolymer, a random copolymer and a block copolymer. The polyimide can also be prepared by separately producing two or more kinds of polyamide acids, mixing these polyamide acids, and heating the mixed polyamide acids to give a polyimide film.
The polyimide resin matrix is preferably produced from a polyimide precursor, namely, polyamide acid or polyamic acid, which can be prepared by reacting in an organic solvent an aromatic tetracarboxylic acid component with an aromatic diamine component in an approximately equimolar ratio at a temperature of lower than approx. 100xc2x0 C., preferably 0 to 60xc2x0 C. The reaction mixture containing the resulting polyimide precursor (i.e. polyamide acid) may contain a small portion of its corresponding polyimide. The reaction mixture containing the polyamide acid is then employed as a dope solution.
The dope solution is casted on an appropriate support and heated to a temperature of 70 to 200xc2x0 C. to give a self-supporting dry film of polyamide acid. The dry film is then separated from the support.
The dry polyamide acid film can be produced by known simultaneous coextrusion of two or more polyamide acid solutions.
The organic solvent employed for the preparation of polyamide acid can be N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, dimethylsulfoxide, hexamethylphosphoramide, or N-methylcaprolactam. These organic solvents can be employed singly or in combination. In the reaction solution, an organic base compound such as imidazole, 2-methylimidazole, 1,2-dimethylimidazole, 2-phenylimidazole, or triethylamine can be incorporated so as to accelerate the desired imidization. The organic base compound can be incorporated into the reaction solution in an amount of 0.1 to 10 weight %, based on the solid content (i.e., polyamide acid) of the solution. Into the reaction solution which can be used as the dope solution, an organic phosphorus compound such as triphenyl phosphite, triphenyl phosphate, or alkyl phosphate can be also incorporated in an amount of 0.05 to 1 weight % (based on the amount of solid content in the reaction solution) so as to facilitate the separation of the dry polyamide acid film from the support. The dope solution may contain an inorganic filler.
On the dry polyamide acid film, which preferably is self-supporting, a palladium compound is placed, for instance, by coating or spraying a solution containing a palladium compound. The coating or spraying solution preferably contains a palladium compound in amount of 0.1 to 5 weight %. The solvent of the solution may be an alcoholic solvent, an acetone solvent, an ethereal solvent, or an amide solvent.
The palladium compound preferably is soluble in the solvent of the dope solution which still remains in the dry polyamide acid film. Examples of the palladium compounds include carboxylic acid salts of palladium, and palladium dihalide. The carboxylic acid preferably contains 6 to 30 carbon atoms, more preferably 6 to 10 carbon atoms, in its molecular structure. Examples of the carboxylic acids include aliphatic acids. Most preferred is a palladium resinate [e.g., Pd(OCOC8H17)2], which is available from Degussa AG.
The coated or sprayed Pd compound-containing solution is permeated into the surface layer of the polyamide acid film.
The polyamide acid film having been coated or sprayed with a Pd compound-containing solution is then heated to a temperature of 370xc2x0 C. or higher, preferably a temperature of 420 to 520xc2x0 C., preferably for 2 to 30 min., to convert the polyamide acid to polyimide. Thus prepared polyimide film preferably has a thickness of approx. 7 to 125 xcexcm, more preferably approx. 10 to 125 xcexcm. The palladium compound permeated into the surface layer is generally converted into palladium metal or other palladium compound. A ratio of thickness between the surface layer containing the palladium metal or palladium compound and other portion of the resulting polyimide film containing neither palladium metal nor palladium compound preferably is 0.05:100 to 2:100.
Alternatively, the palladium compound can be incorporated into the polyamide acid film by simultaneous extruding method, for instance, extruding an aromatic polyimide precursor solution containing no palladium compound and an aromatic polyimide precursor solution containing a palladium compound from a die simultaneously and placing one extruded precursor solution on the other precursor solution to give a combined precursor solution film. The simultaneous extrusion of polyamide acid solutions is described in Japanese Patent Publication H7-102661. The coextruded polyamide acid solutions are dried and heated to a temperature of 370xc2x0 C. or higher, preferably a temperature of 420 to 520xc2x0 C., preferably for 2 to 30 min., to convert the polyamide acid to a polyimide, so that a polyimide film having a surface layer which contains palladium metal or a palladium compound.
On the polyimide film having a surface layer which contains palladium metal or a palladium compound, a metal film is formed by chemical plating. There is no specific limitation with respect to the chemical plating procedure, and known procedures are employable. For instance, the surface layer of the polyimide film is washed with water, treated with a diluted sulfuric acid (for activation of Pd or acceleration), heated to dryness, allowed for cooling to room temperature, washed again with water, immersed in a plating solution for approx. 10 to 60 min. to plate the surface layer with metal, washed again with water, and dried. The plating solution can be a known chemical plating solution which contains a salt of copper, nickel, tin, gold, silver, or one of their mixture. The plated metal preferably has a thickness of approx. 0.1 to 20 xcexcm.
On the plated metal layer of the polyimide film can be formed an electrolytically plated metal layer. A known electrolytical plating method can be employed. For instance, the polyimide film having the chemically plated metal layer is immersed in an electrolytic metal salt-containing aqueous acidic solution to form a negative electrode, and a positive electrode is then placed in the solution. An electric current is flowed between these electrodes to deposit metal on the chemically plated metal layer of the polyimide film. The electrolytic metal salt-containing aqueous acidic solution may comprise 50 to 200 g/L (specifically 50 to 100 g/L) of a metal salt such as copper(II) sulfate and 100 to 300/L of sulfuric acid. If desired, a small amount of a brightener can be incorporated. The electrolytic plating can be carried out at a temperature of 20 to 30xc2x0 C., an negative electrode current density of 2 to 8 A/dm2, a negative electrode efficiency of 95 to 100%, a positive/negative surface area ratio of 1:1, and at a voltage of 6 V or lower, under airy stirring and continuous filtration, using a copper pole as a negative electrode. The metal layer (e.g., a layer of copper, nickel, tin, gold, silver, or one of their mixtures) electrolytically plated on the surface layer of the polyimide film preferably has a thickness of 1 to 30 xcexcm.
Thus electrolytically plated metal layer is bonded to the surface layer of the polyimide film with high bonding strength and is hardly separated from the polyimide film.
Therefore, the metal-plated polyimide film according to the invention can be favorably employed as material for the preparation of electronic devices such as MCM (Multi Chip Module) and FPC (Flexible Printed Circuit Board).
The present invention is further described by the following Examples.